With the development of electronic technology, more and more portable electronic products, such as MP3 players, mobile telephones and digital camera etc., are put into use in the daily life Generally, high end electronic products are supplied by a battery with high energy storing performance. At present, only the charging power source which matches the corresponding portable device or possesses higher load capacity and better load fluctuation features can be used due to the fact that different power sources, e.g., universal serial bus USB power source or transformer etc., own different load capacities and load fluctuation features.
In an existing technology 1, an approach of constant current-constant voltage is used to charge the battery. Namely, a constant charging current is used for charging the battery when the voltage of battery is lower than 4.2V. This process is called the phase of constant current. The charging power source outputs 4.2V constant voltage when the voltage of battery is equal to 4.2V. At the same time, the charging current decreases gradually due to the battery features. This process is called the phase of constant voltage. In the phase of constant current, the charging current keeps unchanged. For the same portable electronic product, however, the unchanged charging current will not vary with the load capacity of the charging power source, so the charging power source with a load capacity lower than the charging current will be unable to charge the battery and the charging power source with a load capacity higher than the charging current can not be effectively utilized. Meanwhile, during the instantaneous increase of the current, the voltage of the charging power source also falls. When the load transient response feature of the charging power source is weak, namely, the voltage of the charging power source falls too much with the change of current, even lower than the voltage of battery, it will make the charging failure.
In another existing technology 2, a charging device, which can adjust the charging current, is used for charging the battery. FIG. 1 shows a logical structure among the charging device, a battery and the charging power source, including: a charging device 1, a spare battery 2, a portable device 3 and a charging power source 4. Among them, a main battery 31 is integrated into the portable device 3. The charging device 1 comprises: a charging integrated circuit 11 for setting the charging current for a spare battery 2 through the resistance; a charging current control module 12 for controlling the resistance to adjust the charging current which the charging integrated circuit 11 provides for the spare battery 2. FIG. 2 shows the charging integrated circuit 11 and a circuit of the charging current control module 12. The charging integrated circuit 11 possesses a current setting terminal ISET, while the charging current control module 12 includes a switching tube Q and two resistances R and r. A terminal b (source/drain electrode) of the switching tube Q is grounded, a terminal a (source/drain electrode) is connected to the current setting terminal ISET of the charging integrated circuit via the resistance R, and a terminal c (grid electrode) is connected to the portable device, for receiving the setting signal of charging current. One terminal of the resistance r is grounded while the other terminal, together with one terminal of resistances R, is connected to the current setting terminal of the charging integrated circuit.
The charging method adopted for the existing technology 2 is that: the charging power source charges the spare battery 2 with a relatively great current through the charging device 1, and then determines whether the portable device 3 is connected with the charging device for charging. When the portable device 3 is connected with the charging device 1 and charged, a charging current setting signal is transmitted to the charging current control module 12 which changes its resistance value according to the charging current setting signal. Finally, the charging integrated circuit 11 lowers the charging current for the spare battery 2 according to the resistance value in the charging current control module 12.
If the charging device provided in the existing technology 2, which can adjust the charging current, is used for charging the battery, it can solve some disadvantages in the existing technology 1 but fail to adjust the charging current to the maximum load capacity of the charging power source. Meanwhile, a great voltage fluctuation will occur on the terminal of the charging power source at the instant that the charging of the spare battery is started for the first time, the portable device is connected suddenly or the charging current of the spare battery is lowered. That is to say, the voltage of the charging power source fluctuates and the fluctuation will cause the instability of the whole device. The voltage fluctuation will also occur in the course of the adjustment of the charging current.